The classic inlet diverter structure for a vertical gas/liquid separator is a spinner. The inlet nozzle can be attached to the vessel at an angle tangential to the circle of the separator shell, or if the nozzle is attached along a radius of the circle, a diverter structure inside the vessel diverts the entire stream 45 to 90.degree. to one side of the separator. Either structure creates a single tangential flow that produces a spiral flow of inlet fluids.
The typical oilfield, vertical, gas well separator receives the inlet flow near a point intermediate the top and bottom of the vessel. Gas exits the top and oil exits the bottom. The objective is that the outlet gas contain no liquid and the outlet liquid contain no gas. To attain these purities, several separation phenomena must occur and several carryover phenomena must be prevented:
(1) A primary separation of bulk liquids from the gas stream must occur before the gas enters the mist extractor. Mist extractors cannot handle heavy liquid loads. If loaded too heavily, liquid won't drain down through the mist extractor, it will be dragged up and carried out by the gas - "re-entrainment".
(2) A mist extractor (typically wire mesh) is required to capture the small particles of liquid in the gas flow. This mist is called entrainment.
(3) In order to attain the highest rate possible through wire mesh without causing re-entrainment, the gas flow profile into the mesh pad must be as uniform as possible. If there are any local areas where the velocity is greater than the average, superficial value, then premature re-entrainment will occur at that point. In the prior art, a single spiral of the input fluids creates a ring of flow that leaves the central portion of the mesh pad unused.
(4) The liquid gravitates to the bottom of the separator. However, if the gas spiral velocity is too high, gravity won't be able to hold the liquid down. Violent waves can form on the liquid surface. The gas tears liquid from the tips of waves, or the suction at the center of the spiral picks up liquid directly, - "re-entrainment".
(5) The liquid phase contains bubbles of liquid that must rise to the surface and rupture. The liquid spins in response to the gas spiral and this reduces the degassing of the liquid. Bubbles rise slowly in moving liquid. Also, if the liquid is slightly foamy, the turbulence at the surface inhibits drainage of foam and even creates foam. Foam is easily picked up by the gas and thrown into the mist extractor. Foam decays slowly in the mist extractor and is easily blown through it.
It quickly becomes clear that separators are not "low technology" structures; they are complex. Separators are sized by using the deceptively simple K-factor formula: ##EQU1## This formula defines the maximum gas velocity that can be allowed without causing re-entrainment. The K-factor is a ratio of kinetic gas force to gravitational force on the liquid. Gravity attempts to keep oceans, cars, and houses attached to the earth, but in a hurricane, the K-factor of the winds is high enough to overcome gravity and pick up objects. The classic K-factor value for vertical separators is 0.35, but this can vary considerably, depending on design. Superior designs allow higher K-factors and produce greater outlet stream purities. Higher K-factors mean smaller vessels and lower cost.
The inlet device affects all 5 separation phenomena and the K-factor which can be used. It has the awesome task of reducing pipeline K-factors of 10 to 20 to a vessel K-factor of 0.35. That is a reduction of velocity by 60 times. That is an absorption of 99.97% of the inlet energy.
There has been a popular misconception floating around within the oil industry that the velocity of the single undivided inlet spiral should be high for efficient separation of gas and liquids. This is only partially true. A higher velocity increases the centrifugal separation ability of the inlet spiral. But for the typical oilfield case, the mist is created due to high pipeline velocities. Using a nozzle that is smaller than the pipeline has a net effect of increasing the amount of mist entering the mist extractor. Additionally, the inlet spiral is not an efficient mist extractor. It catches particles above 100 microns while the wire mesh catches particles above 5 microns. Increasing the spiral velocity is not only unnecessary, it is harmful. It increases re-entrainment due to the other four phenomena mentioned above. An inlet structure should reduce velocity, not increase it. Two exceptions are the genre of centrifugal separators designed to catch very fine mist produced from rapid condensation, or for cyclone polishing separation. But this misapplied concept has floated through the past century inspiring numerous ill-fated failures.
A means is needed to reduce the velocity of the incoming well stream to improve the separation of gas and liquids by giving a more efficient primary separation of bulk liquids, a more uniform flow profile into the mist extractor, less liquid pickup from the liquid surface, and faster degassing and foam decay.